As a technology for evaluating a hardness of a material, a method using an ultrasonic wave is known. Furthermore, although a difference between hardnesses of the material causes a larger change to a phase of the ultrasonic wave than to a frequency thereof, the accuracy of phase measurement technology is not necessarily high, and in consideration of this the present inventors have contrived a technology which converts the phase change into a frequency change, and have disclosed the technology in Patent Document 1. This technology has a constitution including an oscillator which introduces the ultrasonic wave into the material; an oscillation detection sensor which detects a reflected wave from the material; an amplifier having an input end connected to a signal output end of the oscillation detection sensor; a phase shift circuit disposed between the output end of the amplifier and a signal input end of the oscillator to change the frequency, thereby shifting a phase difference to zero, when the phase difference is generated between an input waveform to the oscillator and an output waveform from the oscillation detection sensor; and a frequency change amount detection means for detecting a frequency change amount for shifting the phase difference to zero. In this constitution, the frequency change amount detection means shifts the phase difference due to the hardness difference to zero, thereby converting the phase difference into a frequency change amount. This conversion uses a previously obtained reference transmission function indicating a relation between an amplitude gain and a phase of the reflected wave with respect to the frequency.
Moreover, as a device contributing to the development of this technology, Patent Document 2 discloses a material property measuring device which includes a sensor comprising pulse introducing means and pulse receiving means. The measuring device analyzes frequencies of an incident wave and a reflected wave thereof, compares a spectral distribution of the incident wave with that of the reflected wave to specify a phase difference θx which is a difference between a phase of the incident wave and a phase of the reflected wave at each frequency fx, and inputs the fx and θx to perform calculation by use of a previously obtained reference transmission characteristic curve, thereby obtaining a hardness of the material from a change df of the frequency when the input θx is set to zero.
Furthermore, Patent Document 3 discloses a device for detecting a hard spot of a living body to which the technologies of Patent Documents 1 and 2 are applied, and the device includes a plurality of probe elements each having an oscillator which introduces oscillation into a biological tissue and an oscillation detection sensor which detects a reflected wave. The probe elements are successively selected by a hardness calculation switch circuit and connected to a hardness calculation unit. It is also disclosed therein that the probe elements are provided with corresponding pressure sensors, respectively, and hardness data of the probe elements at pressing pressures in a predetermined range are two-dimensionally displayed on a display unit to detect a hard spot.
In addition, as a device by inventors other than the present inventor, Patent Document 4 discloses an ultrasonic diagnosis device including an ultrasonic probe provided with an ultrasonic transducer which irradiates a part of a subject to be observed with an ultrasonic wave, and receives an echo signal from the part of the subject to be observed, whereby an ultrasonic image is generated from sound line data obtained by digitizing the echo signal, and it is then displayed. The ultrasonic diagnosis device uses a timing signal synchronized with two points in one cycle of an electrocardiographic signal on the basis of movement of the heart of the subject to acquire sound line data for two frames at the two points, calculates strain of a biological tissue from the sound line data for the two frames having a time difference, and displays an elastic image quantitatively indicating a hardness on the basis of the strain.